GravityTale
by I'mWishingforaStarRod
Summary: Fight or spare. Kill or save. Salvation or destruction. When Mabel Pines fell into the Underground, she had two choices. And the fate of everyone she met depended on what choice she made. (An Undertale/Gravity Falls AU staring Mabel as Frisk! Massive work in progress!)
1. 1: The Fallen Human

The child, a girl named Mabel Pines, grinned up at her mother. She was tucked in her bed very snugly, and her mother was about to leave the room and bid her goodnight, but Mabel had asked a question she had probably asked her parents a million times.

"A story? Mabel, it's late. And growing girls need their sleep."

But Mabel pleaded, bringing out her puppydog eyes, and her mother sighed and relented. She sat down on a desk chair near her daughter's bed, and gave a slight cough before beginning her story.

"All right. I'll tell you a true story, how about that? It's about a huge war that took place many years ago.

Long ago, two races had ruled over Earth: HUMANS and MONSTERS.."

Then Mabel had asked her mother what the monsters looked like, and the mother cracked a wry smile. "That was a big problem for a while, because the humans and monsters looked so much alike! They both looked like ordinary people. But the main difference between the humans and the monsters were the nature of their SOULS."

Mabel had made a noise of interest, and her mom grinned a little proudly and continued. "See, a human soul is so much stronger and powerful than a monster's. A monster's soul would break and disentegrate as soon as they died, but a human's live on, because they have so much more feeling and emotion, unlike the monsters'!"

The twelve year-old girl had to remind her mom to talk about the war. Her mother playfully rolled her eyes. "I was getting to that, be patient sweetie. One day, war broke out between the two races. Each one of them wanted to rule Earth. After a long battle, the humans were victorious! Then they sealed all the monsters underground with a magic spell, and they were trapped there forever."

Mabel, feeling a bit sorry for the monsters, asked if they were still down there. Her mother laughed. "Don't worry darling, they're still down there! They're not going to hurt you! Those mean monsters will never get back up to the surface, I promise you."

With that, her mother smiled, kissed her forehead, and bid her daughter goodnight, shutting off the lights and closing the door behind her.

- ** _GRAVITYTALE_** -

MOUNT EBOTT, 2012

Legends say that those who climb the mountain never return.

But Mabel Pines was willing to test her luck.

She climbed with arduous labor, scaling up rocks and trees fairly easily. She had dressed comfortably for the journey; one of her favorite blue sweaters with two pink stripes, and a simple blue skirt. She wore her best walking shoes and a bright red headband. Yep, she was ready for anything!

Several hours later, however, Mabel was beginning to regret not bringing a water bottle. She was thirsty, and hungry, and she had scraped her knee, but a good bandaid had fixed that. She just needed to find some water.

Mabel gasped a little when she turned a corner on a path she it looked like nobody else had taken and saw a huge gaping hole in the ground, like a bottomless pit! She briefly wondered if it led to the underground where all the monsters were held, like her mom had told her during her bedtime stories. She crept closer, but then, to her horror, she tripped on a vine and fell down, down, down, into the gaping black mass of the bottomless pit.

- ** _GRAVITYTALE_** -

When Mabel awoke, she was lying on a soft bed of golden flowers. She sat up, but then she immediately had to lie back down again. A sudden headrush had sent her back down with an inaudible groan, and until the pain went away Mabel outright refused to get up.

Well, since she could feel pain, she obviously wasn't dead...

Mabel looked up helplessly at the tiny pinprick of a light, or the hole she had fallen into. She sighed. It might take a while to get back up to the surface. She was probably going to be late for dinner.

After a minute or two of thinking Mabel felt better and sat up once more, though more slowly this time. She stood up on wobbly legs, retrieved her fallen red headband, then walked through a surprisingly square corridor and through an open gateway.

It was surprisingly dark in the next room; only lit by an unknown light source shining on a single golden flower, but this one was different.

This one had a _face_.

"Howdy! I'm FLOWEY, FLOWEY the FLOWER!"

It was grinning at Mabel innocently, a happy grin on its somewhat simple face. It cocked its "head" to the side a little and hummed lightly as it looked her over. It finally nodded a little to itself, and Mabel blinked. This was certainly strange... Was she dreaming?

The flower, Flowey, kept talking. "You're new to the UNDERGROUND, aren't ya?"

Mabel nodded.

"Golly, you must be so confused!" The talking flower continued. "Someone ought to teach you how things work around here! I guess little old me will have to do!"

Mabel shrugged her shoulders and agreed to Flowey's idea. What's the worst that could happen?

"Ready?" Mabel shook her head determinedly. "Okay! Here we go!"

Flowey then raised its petals as the girl stepped close to it, and Mabel felt something leave her chest. She looked down in surprise to see a small red heart close to her chest, moving whenever she did. Flowey grinned.

"See that heart? That is your SOUL. The very culmination of your being!"

Mabel gazed at her soul and then at Flowey, mouth open. A million questions sprang up in the human's thoat but Flowey was quick to interrupt.

"Your SOUL starts out weak, but can grow strong if you gain a lot of LV." Mabel looked confused and asked what LV stood for. Flowey gave her a slightly condescending look.

"What's LV stand for? Why, LOVE, of course! You want some love, don't you?"

Mabel, distracted from her previous questions, nodded eagerly. Of course she wanted love! Didn't everybody?

"Don't worry," Flowey continued, "I'll share some with you!" Then the flower gave a wink, and Mabel let out a small giggle. But the giggle abruptly ended when a bunch of small white pellets appeared out of nowhere, startling Mabel and making her instinctively step back. Flowey looked unfazed.

"Down here, LOVE is shared through...little white...'Friendliness pellets.'" Flowey looked away from her and Mabel gave the golden flower a confused look. That didn't make much sense, but Mabel just decided to roll with the punches and see what would happen.

Flowey looked back at the human girl. "Are you ready? Move around! Get as much as you can!" He dropped the pellets, sending them streaming towards Mabel, who eagerly rushed forward to catch them, but as soon as her hand touched a friendliness pellet a white hot pain shot through her body and she stumbled backwards. What was THAT?!

Suddenly feeling very weak, Mabel looked up at Flowey again and felt a jolt when the flower had exchanged it's jolly expression with a menacing fanged smile. " ** _You idiot._** " It said, in its high-pitched voice. " ** _In this world, it's KILL OR BE KILLED."_**

Mabel felt more betrayed than angry at the flower, but her jaw dropped in shock once more as a ring of the not-so-friendly pellets surrounded her, effectively trapping her inside ** _. "Why would ANYONE pass up an oppurtunity like this?!"_** The evil flower crowed, and to her horror Mabel saw that the ring was growing ever smaller. She was trapped. She was going to-

 _ **"DIE.**_ "

Flowey cackled, his soprano voice echoing through the darkened room, as Mabel sat there, trying and failing to figure out a plan of what she could do to keep going, to survive...

But then, a shining ray of hope arrived. Mabel suddenly felt a million times better, as if she had gotten healed, eaten for the first time in forever, or rested. Or all three at once. Flowey stopped and gave the human a questioning look as the pellets suddenly flew away, freeing Mabel.

She was about to dance in happy celebration but was interrupted by an axe, spinning as it was hurled through the air by an unknown figure, and it successfully knocked Flowey away. Mabel cheered, then gasped in shock for what seemed like the umpteenth time as she gazed up at her savior.

"Hey kid, you okay? It was awful for that flower dude to gang up on you like that. Lucky for you I stepped into the scene."

The figure stepped into the light and reached out a pale but sturdy hand. Mabel took it quickly and smiled gratefully at the tall, redheaded teenager that had saved her.

"Gonna head with me? Cool beans! My name's Wendy. Welcome to the Underground!"

 **YOOOOOOO! Welcome everyone to my brand new, top of the line Fanfiction, GravityTale! As you can tell, this is a a crossover between my two favorite Fandoms. Each character from Gravity Falls will represent a corresponding character from Undertale. I.e, Mabel as Frisk and Wendy as Toriel.**

 **I will definitely have a LOT more planned for the future! Hopefully my update for this fic will be at least somewhat faster than RoleSwap Au, haha..**

 **Anyways, thanks a billion for reading! Please make sure to leave a little Favorite for me and to review! I always love to hear from you guys!**

 **Until next time,**

 **Star**

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	2. 2: Battles in the Ruins

Without even bothering to ask the name of the person she just saved, Wendy began to lead Mabel out of the darkened room, breaking the silence with some much-needed explanations. "Good thing I found you, huh? I walk around every day to check if anyone's fallen down." Here she shot Mabel a grin. "Easiest job ever."

Mabel gave a small laugh as she followed behind the redhead. "Weird, though. You're the first person to fall down here in a long time. But that's okay, I've got Ya."

Now in a new room, Mabel found what looked like a glittering golden triangle settled in a large pile of leaves. She walked over to it and poked it softly with her index finger.

 _The shadow of the ruins looms above, filling you with determination._

Mabel smiled at this, feeling better about her situation, and ran through a large doorway to catch up with the monster.

While walking to the next room, Mabel got a good look at her new friend. Wendy was tall, at least a foot taller than her, and she had long red hair that fell past her shoulders. She was pale, but her face and arms were dotted with freckles. She wore a loose-fitting purple shirt with a picture of a weird six-fingered hand on it, and wore tight pants with muddy boots. And from the lumberjack hat on her head and the axe hanging from her belt loop, Mabel decided that Wendy was undoubtably very tough. She was glad the ginger was her friend and not her foe.

This next room was smaller, and contained six stones lines up in a pattern, as well as a sign and a golden door. Wendy gestured towards the stones. "So! Welcome to your new home, man! Make yourself comfortable. Not on those rocks, though. See, in the Ruins, there are tons of rooms filled with puzzles. I know the puzzles like the back of my hand, so just stay close and you'll do great!"

Wendy then rushed forward and stepped on four of the stones, then pressed a switch on the far wall. The door opened with a small click. "See? Back of my hand! Now c'mon to the next room; You're gonna love this one!"

The redheaded monster raced through the door, leaving Mabel to look at the sign on the wall.

"Only the fearless may proceed. Brave ones, foolish ones. Both walk not the middle road."

Mabel nodded seriously and walked through the door, encouraged. She was brave. She could make it home.

"Alright!" Wendy called as Mabel walked up to her. "Now, in this room, you've got to press some switches. It's cool, I've marked them for you. They're the yellow ones." And with that, Wendy walked to the end of the room to wait, leaving Mabel to walk around and press the marked switches. It wasn't hard, which Mabel was grateful for. She was going to need some sleep and nourishment if she was expected to do anything harder than this. She would try, though.

When Mabel had pressed both switches, Wendy cheered. "Woo! You did it! I'm proud of you, kid! All right, now to the next room!"

The human beamed in pride as Wendy led her through some disarmed traps and to the next room. The one contained a wax statue of an old man, wearing a suit and a fez. The statue was giving Mabel a winning smile and a thumbs up. Mabel returned it, and Wendy laughed.

"Okay, but seriously. Since you're a human living here in the UNDERGROUND, monsters are going to want to attack and possibly kill you." Mabel gulped, but tugged on Wendy's sleeve to ask a question.

"How will people know you're human if humans and monsters look alike? Simple. That." Here she pointed at Mabel's chest. To her shock, Mabel realized that the little red heart from earlier was still there. She looked back up at the teenager in amazement. "Yeah, it's pretty neat. You've got a pretty powerful soul, and monsters are going to want to take it from you, and you'll enter a FIGHT. So you're going to have to either SPARE them or just stall for time, and I'll come save Ya." She gestured towards the wax statue. "That's what this is for. For practice! Now, just step up to the statue and start up a nice conversation."

Mabel nodded, understanding, and stepped in front of the wax figure.

ENCOUNTERED WAX STAN!

Mabel decided to follow Wendy's advice and talked to the statue. It doesn't seem much for conversation, but Wendy seemed to be happy. Mabel took that as a win.

"Great job, dude!" Wendy called to the human as she stepped away from the statue. Mabel was slightly disappointed that she hadn't earned anything from the battle, she might need money to buy some snacks, or a human sized hamster ball. But the monster had seemed happy, so it was prize enough. Mabel followed her to the next room.

This next room was long, and had a curved path. "There's another puzzle in this room. Bet Ya you can't figure it out!"

Mabel dashed after Wendy, grinning, ignoring the path. But then then she had to come to a stop when...something jumped in front of her.

SCHMEBULOCK ATTACKS!

This... "Schmebulock"... Was a very tiny person, with unfocused eyes and a scraggly gray beard. He wore a red hat and overalls, and had several teeth missing. Mabel wondered what she should do.

Her first thought was to talk, like Wendy had advised her. So she did just that, offering the little person a nice compliment.

The...Schmebulock blushed and smiled widely, but had a slightly confused face. It was if the small being didn't understand what Mabel had said, but was flattered anyway.

The tiny thing readied a stance and looked like it was about to attack in reply when Wendy walked up, hands on her hips. She gave Schmebulock a glare, sending a shiver down Mabel's spine, and made sure the small thing was looking when Wendy threateningly ran her fingers over the sharp blade of her ax.

Needless to say, Schmebulock left immediately.

Mabel made sure to thank her protector politely as they carried on, but had to ask Wendy just what that creature was.

"Oh, that thing? It was a gnome." Wendy said offhandedly, and started to explain more when the human gave her a questioning look. "There are mostly monsters that live down here in the Underground, but there are some indigenous species that live here too. I think they're as eager to escape this place as we are.

There are actually a ton of different species. Merpeople, gnomes, ghosts, spider people, demons, you know, that kinda thing."

She abruptly stopped speaking at the mention of a demon and upped her pace, and leaving Mabel to check another sign.

"The western room is the eastern room's blueprint." A puzzle! Mabel grinned at the realization and turned to the path, making sure to memorize it. She stepped into the next room with courage, but her expression faltered slightly when she saw a long rectangle of sharp spikes arrayed in front of her. She gripped Wendy's hand tightly when she thought she saw dried blood on one of the spikes.

Wendy looked down at the small human with pity, noticing the way her SOUL beat quickly, like a frightened heart. She grilled Mabel's hand back firmly, and led her through the spikes with no difficulty. When Mabel gave the taller redhead a questioning look, Wendy hid her new fond feelings for the child and said, "Well, puzzles seemed a little too dangerous for right now. I want you to make it to my house in one piece."

Mabel smiled at these words and swung their hands a little as they walked, feeling calm for the first time since her fall to the Underground. Everything was going to be okay, she would be able to go home, because Wendy was going to help her. It was a comforting thought.

The next room was a long hallway, so long Mabel could barely see the end of it. Here, Wendy stopped, and turned to the small human.

"Well dude, you've done really well so far! I'm proud of you!" Mabel blushed and grinned, showing metal braces. "But," Mabels heart sank, "I have to request something from you. It's not very hard, don't worry. All I want you to do is walk to the end of the room on your own."

And with that, Wendy ran off.

The human girl didn't much like this exercise, partly because she was afraid a monster would hurt her while Wendy was wherever she was, so she upped her pace and basically ran to the end of the room. There was a large column at the end of the hallway, and as soon as Mabel reached the end of the room, Wendy walked out from behind it. Mabel jumped in surprise, and Wendy laughed.

"See, look at that! You're doing so well, dude! Now, I'm sorry I worried you, but I gotta do this again. I got some stuff to do and take care of, so I'm going to have to leave you here for a little bit. I won't be too long, hopefully. I have an ex-boyfriend that lives around here, and he will literally talk to me for hours, it's so freakin' annoying. I wish he would move on already..."

Mabel gave the monster a confused look, and Wendy quickly waved a hand to dismiss the subject. "Eh, whatever. It's too dumb for you to care about, I promise. You just gotta stay here! Look, I'll even give you a CELL PHONE."

The girl smiled at the old flip phone she had been given. That was nice of her protector. Wendy grinned, putting a hand on her hip. "My numbers on there. Just hit me up if you need anything. So, I'm out! _Peace_!"

Wendy sent Mabel a "locked lips" gesture, and then sprinted away, leaving Mabel on her own.

 **Hello everyone! Sorry this chapter is mostly exposition, but I had to have a setting going and everything. And thank you all so much for all the likes and comments! They mean so much to me, you don't know. I'm sorry, but I'm not going to reveal who each character is yet, you're going to have to wait and find out ;) , but I absolutely LOVE to hear your theories about who each character is and what will happen! Seriously, I will be your best friend and love you forever if you do that.**

 **Thanks again and see you soon!**

 **-Star**

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